Magnetic tunnel junction for MRAM applications

ABSTRACT

A MTJ in an MRAM array is disclosed with a composite free layer having a lower crystalline layer contacting a tunnel barrier and an upper amorphous NiFeX layer for improved bit switching performance. The crystalline layer is Fe, Ni, or FeB with a thickness of at least 6 Angstroms which affords a high magnetoresistive ratio. The X element in the NiFeX layer is Mg, Hf, Zr, Nb, or Ta with a content of 5 to 30 atomic %. NiFeX thickness is preferably between 20 to 40 Angstroms to substantially reduce bit line switching current and number of shorted bits. In an alternative embodiment, the crystalline layer may be a Fe/NiFe bilayer. Optionally, the amorphous layer may have a NiFeM 1 /NiFeM 2  configuration where M 1  and M 2  are Mg, Hf, Zr, Nb, or Ta, and M1 is unequal to M2. Annealing at 300° C. to 360° C. provides a high magnetoresistive ratio of about 150%.

This is a Divisional application of U.S. patent application Ser. No.12/930,877, filed on Jan. 19, 2011, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. No. 7,528,457;U.S. Pat. No. 7,595,520; U.S. Pat. No. 7,663,131; and U.S. Pat. No.7,672,093; assigned to a common assignee and herein incorporated byreference in their entirety.

FIELD OF THE INVENTION

The invention relates to a high performance Magnetic Tunneling Junction(MTJ) element and a method for making the same, and in particular, to acomposite free layer comprised of a crystalline layer that interfaceswith a tunnel barrier layer and an amorphous layer contacting anopposite surface of the crystalline layer with respect to the tunnelbarrier layer to improve bit switching characteristics while maintaininga high magnetoresistive (MR) ratio.

BACKGROUND OF THE INVENTION

Magnetoresistive Random Access Memory (MRAM), based on the integrationof silicon CMOS with MTJ technology, is a major emerging technology thatis highly competitive with existing semiconductor memories such as SRAM,DRAM, Flash, etc. A MRAM device is generally comprised of an array ofparallel first conductive lines on a horizontal plane, an array ofparallel second conductive lines on a second horizontal plane spacedabove and formed in a direction perpendicular to the first conductivelines, and an MTJ element interposed between a first conductive line anda second conductive line at each crossover location. A first conductiveline may be a word line while a second conductive line is a bit line orvice versa. Alternatively, a first conductive line may be a bottomelectrode that is a sectioned line while a second conductive line is abit line (or word line). There are typically other devices includingtransistors and diodes below the array of first conductive lines as wellas peripheral circuits used to select certain MRAM cells within the MRAMarray for read or write operations. A high speed version of MRAMarchitecture consists of a cell with an access transistor and a MTJ(1T1MTJ) in the array.

A MTJ element may be based on a tunneling magneto-resistance (TMR)effect wherein a stack of layers has a configuration in which twoferromagnetic layers are separated by a thin non-magnetic dielectriclayer. In a MRAM device, the MTJ element is formed between a bottomelectrode such as a first conductive line and a top electrode which is asecond conductive line. A MTJ stack of layers that is subsequentlypatterned to form a MTJ element may be formed in a so-called bottom spinvalve configuration by sequentially depositing a seed layer, ananti-ferromagnetic (AFM) pinning layer, a ferromagnetic “pinned” layer,a thin tunnel barrier layer, a ferromagnetic “free” layer, and a cappinglayer. In a MRAM MTJ, the free layer has traditionally been made of NiFebecause of its reproducible and reliable switching characteristics asdemonstrated by a low switching field (Hc) and low switching fielduniformity (σHc).

The pinned layer has a magnetic moment that is fixed in the “y”direction, for example, by exchange coupling with the adjacent AFM layerthat is also magnetized in the “y” direction. The free layer has amagnetic moment that is either parallel or anti-parallel to the magneticmoment in the pinned layer. The tunnel barrier layer is thin enough thata current through it can be established by quantum mechanical tunnelingof conduction electrons. The magnetic moment of the free layer maychange in response to external magnetic fields and it is the relativeorientation of the magnetic moments between the free and pinned layersthat determines the tunneling current and therefore the resistance ofthe tunneling junction. When a sense current is passed from the topelectrode to the bottom electrode in a direction perpendicular to theMTJ layers, a lower resistance is detected when the magnetizationdirections of the free and pinned layers are in a parallel state (“1”memory state) and a higher resistance is noted when they are in ananti-parallel state or “0” memory state.

An alternative MRAM technology has been developed in the past severalyears and is called spin-transfer MRAM or STT-RAM. The spin-transfereffect arises from the spin dependent electron transport properties offerromagnetic-spacer-ferromagnetic multilayers. When a spin-polarizedcurrent transverses a magnetic multilayer in a current perpendicular toplane (CPP) configuration, the spin angular moment of electrons incidenton a ferromagnetic layer interacts with magnetic moments of theferromagnetic layer near the interface between the ferromagnetic andnon-magnetic spacer. Through this interaction, the electrons transfer aportion of their angular momentum to the ferromagnetic layer. As aresult, spin-polarized current can switch the magnetization direction ofthe ferromagnetic (free) layer if the current density is sufficientlyhigh, and if the dimensions of the multilayer are small. The differencebetween a STT-RAM and a conventional MRAM is only in the write operationmechanism. The read mechanism is the same.

A high performance MRAM MTJ element is characterized by a high tunnelingmagnetoresistive (TMR) ratio which is dR/R where R is the minimumresistance of the MTJ element and dR is the change in resistanceobserved by switching the magnetic state of the free layer. A high TMRratio and resistance uniformity (Rp_cov), and a low switching field (Hc)and low magnetostriction (λ_(S)) value are desirable for conventionalMRAM applications. For STT-RAM, a high λ_(S) and high Hc leads to highanisotropy for greater thermal stability. This result is accomplished by(a) well controlled magnetization and switching of the free layer, (b)well controlled magnetization of a pinned layer that has a largeexchange field and high thermal stability and, (c) integrity of thetunnel barrier layer. Other important MTJ properties in both MRAM andSTT-RAM are a low bit-to-bit resistance variation, a low number ofshorted bits, and a low bit-to-bit switching variation. Simultaneousoptimization of all the aforementioned parameters is necessary formaking high performance MRAM products.

Numerous MTJ designs have been proposed and fabricated in the prior artbut there is still a need for improvement in all of the performancecategories mentioned above. Typically, an improvement in one propertyleads to a degradation in one or more other parameters. For example,NiFe affords excellent switching properties but the TMR ratio with a MTJhaving a NiFe free layer is lower than can be achieved with CoFe, CoFeB,or other free layer materials. Even a CoFeB/NiFe free layer no longerexhibits a high TMR characteristic of MgO/CoFeB configurations becausethe NiFe layer with fcc (111) crystal structure prevents the adjacentCoFeB layer from crystallizing in the bcc (001) phase necessary for highspin polarization (and thus high TMR) at the MgO/CoFeB interface.

A MgO tunnel barrier when used in combination with CoFeB pinned and freelayers is known to yield a high TMR ratio. However, CoFeB is known tohave a relatively high magnetoresistance value which is a disadvantage.U.S. Pat. No. 7,750,421 discloses a MgO tunnel barrier and a Fe/CoFeBfree layer thereon comprising a crystalline layer and an amorphous layerto decrease the damping constant in a STT-RAM while maintaining a highMR ratio.

In U.S. Pat. No. 7,666,467, a MTJ is described having a MgO tunnelbarrier, a thin interface layer of crystalline CoFe, and an amorphousferromagnetic layer to form a composite free layer.

U.S. Pat. No. 7,738,287 teaches that a ferromagnetic free or pinnedlayer with a crystalline property may be modified to have an amorphouscharacter by including elements such as Zr, Hf, Nb, and Ta.

However, an improved MTJ is still required that improves bit lineshorting and switching properties while maintaining a high TMR ratio andwithout compromising other important MTJ parameters.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide an improved MTJelement that enhances the bit switching characteristic of a MRAM suchthat the number of unswitched bits at a given bit line current isreduced compared with the prior art.

A second objective of the present invention is to achieve the improvedbit switching property of the first objective without degrading the MRratio of the MTJ.

A third objective of the present invention is to simultaneously achievea reduced number of shorted bits and enhanced bit switching propertyincluding a lower switching current without degrading the MR ratio ofthe MTJ.

According to a first embodiment, these objectives are achieved with aMTJ element comprised of at least a pinned layer/tunnel barrierlayer/free layer configuration wherein a composite free layer adjoins asurface of the tunnel barrier layer which is preferably MgO. A keyfeature is the composite free layer that has a crystalline ferromagneticlayer adjoining the tunnel barrier layer and an amorphous layercontacting an opposing surface of the crystalline layer with respect tothe MgO/crystalline layer interface. In particular, the crystallinelayer preferably has a (001) bcc structure that leads to a high TMRratio while the amorphous layer improves bit switching and bit shortingcharacteristics. For example, a Fe/NiFeHf composite free layer isespecially suited to realize the objectives and advantages of thepresent invention. However, the NiFeHf layer should be in the range of20 to 40 Angstroms thick to provide the full advantage of a reducednumber of shorted bits and a substantially lower bit switching current.In an alternative embodiment, Hf may be replaced by X=Mg, Ta, Zr, or Nb.Preferably, the NiFeX layer has a sufficiently high X content to providean amorphous structure.

In a second embodiment, the bilayer configuration of the composite freelayer in the first embodiment is replaced by a trilayer with aFe/NiFe/NiFeX composition wherein both of Fe and NiFe are crystallinelayers and NiFeX is an amorphous layer in which X is Hf, Mg, Ta, Zr, orNb. Alternatively, the amorphous layer may be made of more than onelayer such as a bilayer configuration represented by NiFeM₁/NiFeM₂ whereM1 and M2 are one of Hf, Mg, Ta, Zr, or Nb, and M₁ is unequal to M₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a MTJ structure having a compositefree layer formed according to a first embodiment of the presentinvention.

FIG. 2 is a cross-sectional view of the MTJ structure having a compositefree layer formed according to a second embodiment of the presentinvention.

FIG. 3 is a cross-sectional view of the MTJ structure having a compositefree layer formed according to a third embodiment of the presentinvention.

FIG. 4 is a table that shows the number of shorted bits on a wafercomprised of a plurality of MTJ elements arranged in rows and columnsand having a structure according to an embodiment of the presentinvention in part (A), and a similar table in part (B) for a prior artMTJ structure.

FIG. 5 is a plot showing the number of unswitched bits (error count) asa function of bit line current for a 4 Mb MRAM chip wherein each MTJ hasa structure according to an embodiment of the present invention.

FIG. 6 is a plot showing the number of unswitched bits (error count) asa function of bit line current for a 4 Mb MRAM chip wherein each MTJ hasa prior art structure similar to the one used to generate the plot inFIG. 4 part (B).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to magnetic tunnel junctions (MTJs) inMRAM devices including STT-RAM. Although a bottom spin valve structureis shown in the exemplary embodiments, the present invention alsoencompasses top spin valve and dual spin valve configurations that havea tunnel barrier sandwiched between a pinned layer and a free layer. Atop surface of a layer is defined as a surface formed in an (x, y) planeand facing away from the substrate. The drawings are provided by way ofexample and are not intended to limit the scope of the invention.Moreover, the drawings are not necessarily drawn to scale and therelative sizes and shapes of various layers may differ from those in anactual device.

It should be understood that a MRAM structure is part of an MRAM arrayin which multiple parallel word lines are formed in a first conductivelayer and multiple top conductor electrodes such as parallel bit linesare formed in a second conductive layer above an array of MTJs.Alternatively, the first conductive layer may be parallel bit lineswhile the second conductive layer is comprised of parallel word lines.The word lines and bit lines are aligned orthogonal to each other and abottom conductor layer may be used to connect each MTJ element with atransistor in the substrate. An MTJ element is typically formed betweena bottom conductor layer and bit line at each location where a bit linecrosses over a word line. Only one MTJ is depicted in the exemplaryembodiments in order to simplify the drawings and direct attention tothe key feature of the present invention which is a composite free layerin a MTJ stack of layers having at least a pinned layer/tunnelbarrier/free layer configuration.

Referring to FIG. 1, a MTJ structure is illustrated according to a firstembodiment of the present invention. The substrate 10 may be a bottomconductor layer, for example, having a thickness in the z-axis directionand with a top surface in the x,y plane. A MTJ stack of layers is nowformed on the substrate 10. It should be understood that all layers inthe MTJ stack may be formed in the same process tool such as an AnelvaC-7100 thin film sputtering system or the like which typically includesthree physical vapor deposition (PVD) chambers each having 5 targets, anoxidation chamber, and a sputter etching chamber. At least one of thePVD chambers is capable of co-sputtering. Typically, the sputterdeposition process involves an argon sputter gas and the targets aremade of metal or alloys to be deposited on a substrate. All MTJ layersmay be formed after a single pump down of the sputter system to enhancethroughput.

In the exemplary embodiment, the MTJ stack of layers is fabricated onthe substrate 10 by sequentially forming a seed layer 11, AFM layer 12,synthetic anti-ferromagnetic (SyAF) pinned layer 13, tunnel barrierlayer 14, composite free layer 17, and a cap layer 18. The seed layer 11may be a layer of NiCr, NiFe, or NiFeCr, for example. In an embodimentwherein the seed layer is grown on a bottom conductor with an amorphousTa capping layer, a smooth and dense (111) seed layer structure resultsthat promotes smooth and densely packed growth in subsequently formedMTJ layers.

The AFM layer 12 is preferably made of MnPt although IrMn, NiMn, OsMn,RuMn, RhMn, PdMn, RuRhMn, or MnPtPd are also acceptable. The SyAF pinnedlayer 13 has an AP2/Ru/AP1 configuration wherein the AP2 layer is formedon the AFM layer 12 and is preferably comprised of CoFe although otherferromagnetic layers are acceptable. The magnetic moment of the AP2layer is pinned in a direction anti-parallel to the magnetic moment ofthe AP1 layer. A slight difference in thickness between the AP2 and AP1layers produces a small net magnetic moment for the SyAF pinned layer 13in an in-plane direction. Exchange coupling between the AP2 layer andthe AP1 layer is facilitated by a coupling layer that is preferablycomprised of Ru with a thickness of about 7.5 Angstroms although Rh orIr may be used instead of Ru. The AP1 layer on the coupling layer may becomprised of CoFe, CoFeB, or combinations thereof.

Above the SyAF pinned layer 13 is formed a thin tunnel barrier layer 14which in the preferred embodiment is made of MgO although AlOx, TiOx, orother tunnel barrier materials used in the art are also acceptable. AMgO tunnel barrier may be formed by depositing a first Mg layer,oxidizing by a radical oxidation method (ROX) or natural oxidation (NOX)method, and then depositing a second Mg layer. After a subsequentannealing step, the tunnel barrier essentially becomes a uniform MgOlayer.

A key feature of the present invention is the composite free layer 17formed on the tunnel barrier 14. The composite free layer has a lowercrystalline magnetic layer 15 contacting a top surface of the tunnelbarrier layer and an upper amorphous layer 16 that may be eithermagnetic or non-magnetic. According to a first embodiment, thecrystalline magnetic layer 15 is made of Fe, an alloy thereof such asFeB_(Y) where y is from 0 to about 5 atomic %, or Ni. The crystallinemagnetic layer 15 has a (001) crystal orientation to match that of theMgO tunnel barrier layer 14 and thereby promote coherent tunneling whichleads to a high TMR ratio. A small amount of B of up to about 5 atomic %may be added to Fe in order to lower Hc and improve thermal stability.Although the present invention also anticipates that CoFe may be used asthe crystalline magnetic layer 15, the switching properties of a MTJcontaining CoFe in the free layer are typically not as favorable as whenCoFe is replaced by Fe.

Upper amorphous layer 16 is employed to improve one or more of switchingperformance and other MTJ characteristics without adversely affectingthe TMR ratio or properties of the lower crystalline layer 15.Preferably, the upper amorphous layer 16 has a thickness from 20 to 40Angstroms and is comprised of NiFeX where X is one of Hf, Zr, Nb, Ta, orMg with a content of about 5 to 30 atomic %. Layer 16 is preferablyamorphous rather than crystalline so that an upper free layer (ifcrystalline with a fcc structure) could not affect the crystal structurein the lower free layer where a (bcc) orientation is preferred for highMR ratio. We have previously disclosed in U.S. Pat. Nos. 7,528,457,7,595,520, 7,663,131, and 7,672,093 how a NiFeX capping layer may beused with various tunnel barrier/free layer configurations to enhancethe TMR ratio of a MTJ through an oxygen gettering effect. In U.S. Pat.No. 7,672,093, a NiFeHf capping layer is formed on a NiFe composite freelayer comprised of two NiFe layers whose magnetostriction constants areof opposite sign. Now we have discovered that a composite free layerincluding an upper amorphous NiFeX layer offers additional benefits byreducing the number of shorted bits and lowering the bit switchingcurrent. Furthermore, the thickness of the crystalline magnetic layer 15should be at least 6 Angstroms in order to maintain a high TMR ratiowhile the upper thickness limit of the Fe, Ni, or FeB crystalline layerdepends on the Mst (magnetic saturation×thickness) requirement for thefree layer. Note that the amorphous layer 16 generally contributessubstantially less toward the Mst requirement for the composite freelayer than the crystalline magnetic layer 15 because the magnetic momentof the former is reduced by the presence of the X element.

The minimum X content necessary to achieve an amorphous NiFeX film isabout 5 atomic % for X. On the other hand, the X content should not bemore than about 30 atomic %, especially for Ta or Nb, to prevent X fromdiffusing into the crystalline magnetic layer 15 and degrading the freelayer properties. It should also be understood that the magnetostriction(λ) of the free layer 17 may be adjusted by changing the thicknesses ofthe layers 15, 16, and by modifying the X content in NiFeX. Althoughboth Fe and NiFeX, for example, contribute a positive value to λ forfree layer 17, Fe provides the larger component. As the % X in NiFeXincreases, λ generally increases. Those skilled in the art willappreciate that NiFe may have either a (+) or (−) λ value, depending onthe Ni content in the alloy. Likewise, a NiFeX alloy may have either a(+) or (−) λ value, depending on the Ni content and X content in thealloy. Typically, a λ less than about 1×10⁻⁶ is desirable for the freelayer 17.

The capping layer 18 formed on the free layer 17 may have a Ta or Ta/Rucomposition although other capping layer materials may be used. Thecapping layer serves as an electrical contact with an overlying topelectrode (bit line) and typically is employed as an etch stop and/orchemical mechanical polish (CMP) stop layer during subsequent processingsteps. A Ta capping layer 18 serves as an oxygen getter layer to preventoxygen from diffusing into the crystalline magnetic layer and loweringthe TMR ratio, and is preferably an α-phase Ta layer with a lowresistance.

According to a second embodiment of the present invention as depicted inFIG. 2, the free layer 17 of the first embodiment may be modified toinclude two crystalline magnetic layers 15 a, 15 b. In particular, alower crystalline layer 15 a made of Fe, FeB_(Y), or Ni contacts tunnelbarrier layer 14, and an upper crystalline layer 15 b comprised of NiFeis formed between crystalline layer 15 a and amorphous NiFeX layer 16.Note that unlike a CoFeB/NiFe configuration where NiFe preventsamorphous CoFeB from crystallizing into the desired (001) bcc structure,crystalline layer 15 a is deposited with a (001) bcc structure and isunaffected by the nature of the NiFe crystal lattice. Furthermore, theaddition of a Fe crystalline layer adjoining the tunnel barrier layerenhances the TMR ratio above that realized with a NiFe/NiFeHf free layerconfiguration. The relative thicknesses of the layers 15 a, 15 b, 16 maybe adjusted to satisfy the Mst and λ requirements of the free layer 17.However, crystalline layer 15 a is preferably at least 6 Angstroms thickto provide a high TMR ratio approaching 150% or higher.

For improved flexibility in modifying the NiFeX composition, the NiFeXlayer 16 in a MTJ stack is preferably deposited by co-sputtering NiFeand X targets. In one embodiment, the NiFe target and X target areplaced at alternating positions in a sputter (PVD) chamber. For example,the NiFe target may be placed at position 2 in an Anelva C-7100sputtering chamber while the X target is located at target position 4.Optionally, the NiFe target may be placed at position 1 and the X targetat position 3. In one embodiment, the NiFe target has a Ni content of 80atomic % and a Fe content of 20 atomic % although Ni/Fe ratios otherthan 4:1 may be employed.

It should be understood that the sputter deposition rate of a specificmetal is dependent on the sputter power applied to the target cathode.Thus, the concentration of the NiFeX layer is controlled by the powerapplied simultaneously to the two respective targets. In the examplewhere X is Mg, the Mg deposition rate is faster than the NiFe depositionrate using the same applied power. To compensate for the unequaldeposition rates, a higher forward power is applied to the NiFe targetthan to the Mg target. The preferred deposition method comprisesapplying a forward power of 30 Watts (W) to 80 W, and more preferably 50W, to the Mg target and a forward power of 100 W to 500 W, and morepreferably 200 W, to the NiFe target to deposit a NiFeMg layer at apressure less than about 0.3 mTorr and at an ambient temperature. Thenon-magnetic property, B_(S) (magnetic moment), of the co-sputteredNiFeMg film is measured with a B—H looper. Composition of thenon-magnetic NiFeMg alloy is analyzed with a well known EDS system usingelectron microscopy. The present invention also encompasses anembodiment in which a NiFeX target is sputtered to form a NiFeX layer inthe MTJ stack in cases where X has a sufficiently low atomic % to allowa NiFeX target of sufficient size to be useful in a manufacturingscheme.

It is believed that one important mechanism responsible for achieving ahigh TMR ratio involves gettering oxygen from the crystalline magneticlayers 15 a, 15 b by the NiFeX layer 16. Thus, crystalline magneticlayers are less oxygen contaminated and have higher conductivity,thereby improving dR/R. Although the actual mechanism is not understoodat this time, it is believed that the oxygen gettering power of a NiFeXlayer may be at least partially responsible for an improved switchingproperty and reduced bit shorts as described herein.

Referring to FIG. 3, a third embodiment of the present invention isillustrated and retains all of the features of the first embodimentexcept the amorphous layer is modified to include a bilayerconfiguration having a lower NiFeM₁ layer 16 a contacting a top surfaceof the crystalline magnetic layer 15 (or NiFe layer 15 b) and an upperNiFeM₂ layer interfacing with the capping layer 18 where M₁ and M₂ areHf, Nb, Ta, Mg, or Zr, M₁ is unequal to M₂, and each of M₁ and M₂ isfrom 5 to 30 atomic %. This embodiment provides more flexibility to theamorphous layer design. For example, M₁=Hf may provide the maximumgettering power in a NiFeM₁ layer but another element such as Ta inNiFeM₂ may yield a better advantage in terms of etch resistance. In thisembodiment, the combined thickness of amorphous layers 16 a, 16 b ispreferably in the range of 20 to 40 Angstroms. When the crystallinemagnetic layer is Fe or Fe/NiFe, the Fe layer preferably has a thicknessof at least 6 Angstroms to provide a TMR value approaching 150% orhigher.

The present invention also encompasses an annealing step after all ofthe MTJ layers have been deposited. For example, in the exemplaryembodiment, the MTJ stack of layers is preferably annealed by applying amagnetic field of about 10K Oe in magnitude along the desired in-planemagnetization direction for about 1 to 2 hours at a temperature between300° C. and 360° C., or less preferably for 5 hours at a temperaturefrom 260° C. to 300° C. with a similar applied magnetic field.

After all of the MTJ layers have been deposited and annealing iscompleted, an MTJ element with sidewalls and a top surface having acircular, elliptical, or polygonal shape from a top view (not shown) aspreviously described in U.S. Pat. No. 7,595,520 which is hereinincorporated by reference in its entirety may be fabricated byconventional photoresist patterning and etching techniques. Thereafter,the remainder of the MRAM structure is formed by well known methodsincluding deposition of an interlevel dielectric layer (ILD) adjacent tothe MTJ element and thereby electrically separating the MTJ from otherMTJ elements in the MRAM array (not shown). Typically, a CMP step isperformed to provide an ILD that is coplanar with the top surface of theMTJ. Then a metal layer (not shown) including bit lines, for example, isformed on the ILD and top surfaces of the MTJ elements in the MRAMarray.

An experiment was conducted to determine the performance of a MTJ stackformed on a substrate according to the present invention. The MTJ stackhas a seed layer/AFM layer/pinned layer/tunnel barrier/freelayer/capping layer configuration represented by the following:NiCr45/MnPt150/Co₉₀Fe₁₀24/Ru/Co₆₀Fe₂₀B₂₀23/MgO13/free layer/Ta510 wherethe number following the composition of each layer is the thickness ofthat layer. The MgO layer was formed by depositing a first Mg layer,performing a radical oxidation (ROX) process, and then depositing asecond Mg layer. Each MTJ stack was annealed with an in-plane magneticfield of 10000 Oe for 2 hours at 330° C. The results in Table 1 wereobtained by using a Capres CIPT (current in plane tunneling) tool thatmeasures dR/R on an unpatterned MTJ stack.

TABLE 1 TMR Ratio of MTJs with different free layer (FL) in NiCr45/PtMn150/CoFe24/Ru7.5/CoFeB23/MgO13/FL/Ta510 Example Free layerconfiguration dR/R Embodiment 1 Fe(16.3)/NiFeHf(30) 163% Embodiment 2Fe(6)/NiFe(19)/NiFeHf(30) 152% Reference 1 Fe(16.3A)/NiFeHf(10) 157%Reference 2 Fe(25)/Al(10) 91% Reference 3 NiFe(34)/NiFeHf(35) 77%

In all examples except reference 2, a Ni_(R)Fe_(S)Hf_(T) layer whereR=75, S=10, and T=15 was deposited by co-sputtering a Ni_(R)Fe_(S)target and a Hf target in an Anelva 7100 sputter deposition system. Withregard to the example representing embodiment 2, the NiFe layer has a Nicontent of 88 atomic %. References 1-3 include free layer stackspreviously built by the inventors for other experiments. TMR ratios forthe examples representing embodiments 1 and 2 demonstrate that dR/R isas high as the value in other designs such as reference 1. One advantageof the present invention as found in the embodiments described herein isa dramatic reduction in the number of shorted bits in MRAM devices.

Referring to FIG. 4, the MTJs of Embodiment 1 and Reference 1 above wereincorporated in 4 Mb MRAM chips. The table in FIG. 4 shows the number ofshorted bits per 0.5 Mb section of a 4 Mb chip, measured for multiplechips across an 8 inch wafer. Each number shown by column and rowcorresponds to one chip. Wafer A comprises chips with MTJ structure ofEmbodiment 1 and Wafer B has chips with MTJ structure of Reference 1.Note that a thicker amorphous NiFeHf layer in wafer A results in anorder of magnitude lower shorted bit number than having a thinner NiFeHflayer in wafer B. We have discovered that a NiFeHf thickness of at least20 to 25 Angstroms is required to provide a significant reduction in theshorted bit number. On the other hand, a NiFeHf thickness above 40Angstroms is likely to lead to a lower TMR ratio.

Another important benefit of the present invention not realized inprevious MTJ designs is a substantially improved bit switchingcharacteristic expressed as the number of unswitched bits at a givenbitline current value. In other words, lowering the number of unswitchedbits (error count) is a key improvement. Referring to FIG. 5, errorcount is plotted as a function of current (mA) for the 4 Mb MRAM chipwith MTJ structures of Embodiment 1. The actual data points in the plothave been replaced by two diagonal lines which represent the averageslope of multiple lines (not shown). In FIG. 6, error count is plottedas a function of current for the 4 Mb MRAM chip having MTJ structures ofReference 1. Again, actual data points have been removed and replaced bytwo diagonal lines representing an average slope of multiple lines (notshown). Note that the x-intercept is about 2 mA (or −2 mA) in FIG. 5 andapproximately 3.4 mA (or −3.4 mA) in FIG. 6 which means that asubstantially lower bitline current is required to switch the bits for aMTJ structure of Embodiment 1. Thus, lower switching current translatesinto less power consumption, lower heating, and improved reliability inthe MRAM device comprised of MTJs of the present invention.

In summary, the present invention provides a lower bitline switchingcurrent and a reduced number of shorted bits compared with prior designswhile maintaining a high TMR ratio. Moreover, magnetostriction of about1×10⁻⁶ can achieved by adjusting the relative thicknesses of thecrystalline magnetic layer and amorphous layer in the composite freelayer, by modifying the Ni and X content in the NiFeX amorphous layer,or by including NiFe in the crystalline magnetic layer as describedherein. The co-sputtering of NiFe and X targets allows flexibility indepositing an amorphous NiFeX layer with a range of X content.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A MTJ element in a magnetic device, comprising: at least apinned layer/tunnel barrier layer/free layer configuration wherein thetunnel barrier layer has a first interface with the pinned layer, andthe free layer is a stack of three layers with a Fe/NiFe/NiFeXconfiguration wherein Fe and NiFe are crystalline magnetic layers, theFe layer interfaces with the tunnel barrier layer, and the NiFeX layerwherein X is one of Hf, Zr, Nb, Ta, or Mg, is amorphous and contacts asurface of the NiFe layer that is opposite to an NiFe interface with theFe layer.
 2. The MTJ element of claim 1 wherein the amorphous NiFeXlayer has a composition wherein X has a content between about 5 and 30atomic %.
 3. The MTJ element of claim 1 wherein the Fe layer has athickness of at least 6 Angstroms.
 4. A MTJ element in a magneticdevice, comprising: (a) a synthetic anti-ferromagnetic (SyAF) pinnedlayer formed on a substrate; (b) a tunnel barrier layer on the SyAFpinned layer; and (c) a free layer with a Fe/NiFe/NiFeX configuration onthe tunnel barrier layer, comprising: (1) a Fe/NiFe stack of crystallinemagnetic layers wherein the Fe layer contacts a top surface of thetunnel barrier layer; and (2) an amorphous NiFeX layer contacting a topsurface of the NiFe layer wherein X is one of Hf, Zr, Nb, Ta, or Mg. 5.The MTJ element of claim 4 wherein the amorphous NiFeX layer has athickness between about 20 and 40 Angstroms.
 6. The MTJ element of claim4 wherein the amorphous NiFeX layer has a composition wherein X has acontent from about 5 to 30 atomic %.
 7. The MTJ element of claim 4wherein the Fe layer has a thickness of at least 6 Angstroms.